I’d say the exceptions that vector::at() throws aren’t really intended to be caught by the immediately surrounding code. They are mainly useful for catching bugs in your code. If you need to bounds-check at runtime because e.g. the index comes from user input, you’re indeed best off with an if statement. So in summary, design … Read more
Sounds like a job for std::copy_if. Define a predicate that keeps track of elements that already have been processed and return false if they have. If you don’t have C++11 support, you can use the clumsily named std::remove_copy_if and invert the logic. This is an untested example: template <typename T> struct NotDuplicate { bool operator()(const … Read more
What happens: x is inserted via push_back. One copy occurs: The newly created element is initialized with the argument. my_int is taken over as zero because xs default constructor initialized it so. The second element is push_back‘d; The vector needs to reallocate the memory since the internal capacity was reached. As no move constructor is … Read more
valarray is kind of an orphan that was born in the wrong place at the wrong time. It’s an attempt at optimization, fairly specifically for the machines that were used for heavy-duty math when it was written — specifically, vector processors like the Crays. For a vector processor, what you generally wanted to do was … Read more
Items in a vector must be assignable (or, in more recent versions of the standard, movable). const objects aren’t assignable, so attempting to store them in a vector will fail (or at least can fail — the code is invalid, but a compiler is free to accept it anyway, if it so chooses, though most … Read more
Wow. So many answers that are close, none with all the right pieces. You need both resize (or reserve) and copy_n, along with remembering the original size. auto old_count = xx.size(); xx.resize(2 * old_count); std::copy_n(xx.begin(), old_count, xx.begin() + old_count); or auto old_count = xx.size(); xx.reserve(2 * old_count); std::copy_n(xx.begin(), old_count, std::back_inserter(xx)); When using reserve, copy_n is … Read more
STL vectors perform bounds checking when the .at() member function is called, but do not perform any checks on the [] operator. When out of bounds, the [] operator produces undefined results.
The rate at which the capacity of a vector grows is implementation dependent. Implementations almost invariably choose exponential growth, in order to meet the amortized constant time requirement for the push_back operation. What amortized constant time means and how exponential growth achieves this is interesting. Every time a vector’s capacity is grown the elements need … Read more
You cannot store arrays in a vector or any other container. The type of the elements to be stored in a container (called the container’s value type) must be both copy constructible and assignable. Arrays are neither. You can, however, use an array class template, like the one provided by Boost, TR1, and C++0x: std::vector<std::array<double, … Read more
EDIT: using c++14, the best solution is very easy to write thanks to lambdas that can now have parameters of type auto. This is my current favorite solution std::sort(v.begin(), v.end(), [](auto &left, auto &right) { return left.second < right.second; }); ORIGINAL ANSWER: Just use a custom comparator (it’s an optional 3rd argument to std::sort) struct … Read more